In the construction of bullet resistant window assemblies, a rectangular thick glass unit commonly includes multiple layers of glass and glass clad polycarbonate sheets, and the composition and thickness of the glass unit is selected according to the level of ballistic and bullet resistance desired. The glass unit is supported by various forms of surrounding frame systems which are commonly constructed from sections of aluminum extrusions attached to steel strips or by forming sheet steel into frame sections. Sometimes, the frame defines a chamber which is filled with concrete to provide a higher level of security. However, in the construction of a high security bullet resistant window assembly, it is desirable to have a relatively light weight and compact frame capable of the highest level or rating of bullet resistance, and also a frame which is neat and decorative in appearance and is easy to install along with the multiple layer glass unit. It is further desirable to provide a frame which may be economically constructed and which may be easily modified to accommodate glass units of different thicknesses and different sizes.